Method for the preparation of tinting patterns for plastic sheeting



Oct. 25, 1960 E. E. WOODACRE 2,957,766 METHOD FOR THE PREPARATION OF TINTING PATTERNS FOR PLASTIC SHEETING Filed June 27, 1957 2 Sheets-Sheet 1 F I G. 2

INVENTOR ERNEST E. WOODAC RE BY Ca ATTORNEY Oct 25, 1960 E. E. WOODACRE 2 957 7 METHOD FOR THE PREPARATION OF TINTING 66 PATTERNS FOR PLASTIC SHEETING Filed June 27, 1957 2 Sheets-Sheet 2 FIG.3

TONE DENSITY Fl G. 4 F l G. 5

LL! 0 2 1 E (I) Z II I (D :l

INVENTOR DISTANCE ACROSS BAND ERNEST E. WOODACRE ATTORNEY NIETHOD FOR THE PREPARATION OF TINTING PATTERNS FOR PLASTIC SHEETING Ernest E. Woodacre, Parkersburg, W. Va., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed June 27, 1957, Ser. No. 668,553 1 Claim. (CI. 96-27) The present invention relates to a method for preparing rotogravure printing cylinders which will print predetermined color patterns of the required tonal uniformity and quality on plastic interlayer used for automotive glazing and more particularly to a method for the preparation of photographic tone patterns having predetermined variations in tone or color intensity.

The gradated coloring of plastic sheeting employed in the preparationof safety glass for automotive uses reduces the glare from the sun or other bright lights. This coloring ofrthe safety glass interlayer resin can be accomplished by several methods such as pigmenting or dyeing the sheeting prior to lamination with the glass. Since, however, the tinted glass reduces the vision of the human eye, it is highly desirable to impart to the plastic sheeting a gradated pattern of color or tone in such a manner that the color or tone is deep or intense in those portions of the safety glass where glare reduction is the 1 primary consideration and less dense in areas where vision is the primary consideration, thereby obtaining the best compromise between glare reduction and vision. In order to avoid strain on the human eye, it is necessary to have this variation in tone or color intensity occur in a very gradual manner. Although it has been possible heretofore to calculate those patterns of gradated tinting which are most suitable for the human eye, and which strike the best compromise between vision and glare reduction in each particular application of the safety glass, it has been very difficult to prepare a printing roll which will accurately reproduce such tone patterns on the interlayer sheet.

It is, therefore, the object of the present invention to provide a method for producing uniformly gradated tinting patterns on plastic sheeting, and particularly on such sheeting as employed in the manufacture of safety glass. It is a further object of the present invention to provide a method for preparing plastic sheeting bearing a-colored band which has uniform color intensity in one direction and a predetermined gradated color intensity pattern perpendicular to that direction. It is still another object of the present invention to prepare printing designs for rotogravure printing cylinders which will enable the transfer of these predetermined gradated color patterns to the plastic sheeting.

The objects of the present invention are accomplished by a method which comprises consecutively exposing a photosensitive plate to a uniformly illuminated slot by repeatedly moving said illuminated slot at a uniform speed across the field of view of said photosensitive plate, the length of the slot corresponding to the length of the pattern in the direction of the gradated tone density, the width of the slot varying with the change in tone density desired in accordance with the equation described hereinafter in greater detail, and,in the case of a reverse half-tone negative plate, preparing a reverse half tone positive plate from said exposed plate, exposing through the positive plate a cylinder coated with a photosensitive resist, etching said cylinder, and thereafter printing the dye.

2,957,766. Patented Oct 25, 1.960

plastic sheeting with said cylinder employing a suitable.

In a preferred embodiment of the present invention the photosensitive plate is exposed through a focus-- ing lens, a half-tone cross-slit lens stop and a half-tone diffraction screen to obtain directly on the photosensi tive plate the desired reverse half-tone dot pattern.

Reverse half-tone is defined as a method for making cylinders or plates for intaglio printing in which a continuous tone-image is represented by dots of essentially the same depth but of varying area to give the efiect of diiferent tone values. Tone density of a reverse half tone pattern is related to the size of the dots in a loca-' tion.

The preparation of the said photographic plate containing an image of the desired tone density pattern may be made in various ways, although, in general, itis accomplished by exposure of the photosensitive plate through a moving slot having a silhouette whose shape is determined by the desired tone intensity pattern. It is essential, however, that the photosensitive plate be ex posed only by light emerging from the slot, that the slot move across the field of the photosensitive plate at a constant linear velocity and that repeated exposure of.' the photosensitive plate occurs before any part of the photosensitive plate is completely developed. The motion. of the blind or slot may be periodic, rectilinear, or rotational or a combination of these. The type of photo--'- graphic plate obtained in the process of the present in-' vention is a reverse half-tone in which a constant number of dots per inch are formed on the photosensitive plate. The dots vary in size in accordance with the amount of light to which any particular dot has been exposed. It is possible to employ either a positive or a negative reverse half-tone dot system.

A positive system consists of the photographic plate embodyingrblack dots on a white background, where areas with larger dots would correspond to areas on the printing cylinder which would print more intense tones, whereas the negative system embodies white dots on a black background.

The reverse half-tone negative system requires the preparation of a positive plate from the exposed negative. The positive plate containing a dot pattern identical to that of the negative is then employed in the preparation of the rotogravure printing cylinder. The shape of the lensv stop determines whether a positive or a negative reverse half-tone dot pattern is obtained.

The general equation employed to determine the width of the slot corresponding to the desired tone density is W=ekt (I) wherein W is the width of the slot at any specific location.

5 represents the tone density desired at the corresponding location on the photosensitive plate.

K is a constant which depends on the spectral sensi-- tivity of the photosensitive plate, developing techniques, e.g., solution strength, concentration, temperature, immersion times, etc., and the physical make-up and con? figuration of the camera and associated equipment. The constant K may be calculated from known photographic equations, tables and graphs, or may be experimentally determined by setting up the system to be used and independently measuring the tone density resulting from a known width, and then calculating k employing the above equation.

Employing the established relationship of width and density, the width of the slot required for a desired density may be calculatedby the following equation where W and p and k are as above and where p and W0" are the experimentally established standard values of density and width for the system employed.

For a negative pattern the standard density 5 selected is one which corresponds to a tone density in the shadow end. or low transmission end of the pattern corresponding to the narrow width of the slot.

. In the case where a positive photographic pattern is desired the standard density selected is onewhi'ch corresponds to the light tone or highlight end of the pattern.

The present invention is further illustrated by flne attached drawings wherein Figures 1 and2 represent schematic drawings ,of the equipment employed to produce the desired grad'ated tone pattern. Figure 3 shows tone density across a positive pattern such as may be employed in the process of the present invention; Figures 4 and 5 show the type of silhouettes that are employed to obtain the tone density pattern on the photosensitive plate. The silhouette shown in Figure 4 is employed when a negative reverse half-tone dot system is employed. The silhouette in Figure 5 is employed when a positive reverse half-tone dot system is employed. Figure 6 shows the variation of light transmittance across a band printed with a cylinder to which a pattern such as the one shown in Figure 3 has been transferred;

In accordance with the terms previously defined, the parts of the slot with the narrowest width would correspond to areas of the photosensitive plate which are exposed to least light and hence where the thinnest screen wall and largest white dots are formed. The large white dots correspond to thelarge black dots in the positive pattern which is used for transfer to the cylinder.

Figures 1 and 2 of the attached drawings show a light box 11 which encloses fluorescent arcs or incandescent lamps 12, inside a box which is provided with a diffusing screen 13, so that the frontal face of the box provides illumination that is of uniform intensity over the frontal area. The box is attached to a mechanism 14 which provides a constant speed drive, and an electric motor 15 which causes the box to oscillate laterally along the tracks 17 and 18. Figure 2 shows the running mechanism of the box which is supported by the rollers 19 and 20. A peg 21 is rigidly attached to the chain 22 and projects into a vertical slot 25, on box 11. Movementof the chain and the attached peg 21, in cooperation with slot 25 causes the box to move horizontally along tracks 18 and 19, reversing its direction at each end of the tracks. On the front of the box is attached a black-faced shield 2 3 made from some suitable sheeting such as cardboard into which is cut a generally triangular slotv 24 such as shown in Figures 4 and 5, and having the shape necessary to form the gradated tone pattern desired on a photographic plate. A camera, shown schematically by the photosensitive plate 27, the grid screen 28, the lens stop 29, and the lens is placed in front of the light box and focused by means of the lens 30, and camera position so that the image 26 projects onto the photosensitive plate 27 is the same size as the slot. The photosensitive plate 27 is large enough so that the image of the slot when in motion can trace out a path which is uniform in tone over any predetermined length bearing the desired relationship to the circumference of the cylinder to which the pattern is to be transferred. The camera itself is usually equipped so that a negative reverse half-tone pattern is projected onto the photosensitive plate 27, i.e., the camera is used in conjunction with a half-tone screen 28, and a negative cross slit stop 29. The corresponding positive reverse half-tone dot pattern to be used for transfer to the printing cylinder is then prepared by normal photographic methods.

Another way of preparing the pattern is to use the camera in conjunction with a half-tone screen and a positive cross-slit stop so that a positive reverse half-tone dot pattern is traced onto the photosensitive plat-e. Still another way of preparing the pattern is to use the camera equipped so that a continuous tone pa rn s raced Oh o the photosensitive plate and then to convert this pattern into the necessary half-tone dot form by known photographic methods.

The relationship of slot widths at various locations which correspond to desired tone densities in a typical photographic pattern is shown by the tables below. The results in Table I were pbtained employing a standard gallery camera with a 150 line screen used at a screen separation distance of and employing a negative cross-slit stop. A standard high contrast orthochromatic plate was employed in conjunction with a line developer. Three l4-watt daylight fluorescent lamps behind white difiusing glass were employed for lighting the slot. The plate was exposed by six traverses of the slot across the length of the plate at a speed of A), ft./sec. The results in Table II were obtained employing the same equipment except that a 120 line screen was employed at a screen separation distanceof Values for k are 1.5 employing the system illustrated by Table I and 1.7

for Table II.

Table I Optical Den- Slot Width in Distance From Light Tone End sity Values Inches-150 of Pattern (Small Dots) For Negative Line Screen Patterns 2.51 6. 6 2. l3 3. l. 81 2. 75 1. 55 1.6 1.22 1.0 .88 .6 54 .3 .33 25 (W0) Table II Optical Den- Slot Width in Distance From Light Tone End sity Values Inchesof Pattern (Small Dots) For Negative Line Screen Patterns 2. 51 6. 6 2.13 3. 47 1.81 2. 3 l. 55 l. 32 1.22 0. 6 88 0. 45 54 0. 2 .33 0. 163 (W Other screens with grid sizes and rulings coarser and finer than the two sizes cited in the examples maybe used. This would necessitate a change in some of the conditions and would result in difierent values of k." The examples cited are for the preparation of negative patterns. If it is desired to prepare a positive pattern, the procedure is similar and the expression valid except in this case is the tone density in the highlight or light tone end.

In the preferred method of the present invention, in which a negative reverse half-tone dot pattern is produced from which a positive reverse half-tone dot pattern is later prepared by known photographic methods, the oscillating shield containing the triangular slot permits control of the exposure and thus control of the size of the half-tone dot developed in various areas of the photosensitive plate, since the dots range over a wide scale of sizes as a function'of the time of exposure. It is therefore possible by the repeated motion of this slot, to obtain uniformity of dot size in one direction of the photosensitive plate and to vary the dot size in any desired manner in the perpendicular direction of the photosensitive plate. Since the dot size developed in an area of a negative reverse half-tone dot pattern is proportional to theintensity of color printed by the corresponding area of the pattern transferred to a rotogravure cylinder, the tone pattern of the negative can thus be transferred to the plastic sheeting. p p

The present invention, therefore, provides a method for imposing a predetermined gradated tone intensity pattern on a photosensitive plate, which in turn can be employed to prepare a printing cylinder, used in the printing of plastic sheeting, particularly such plastic sheeting as polyvinyl acetal sheeting which can readily be printed on and which absorbs all the dye on the rotogravure cylinder. The overall tone density of the exposed area of the photosnesitive plate and the uniformity of tone parallel to the motion of the slot will depend on the intensity and uniformity of the light source, the frequency and constant speed of the slot oscillation, and the speed of the photosensitive plate. Since the intensity of the light will affect the density of the photosensitive plate, and since, in the present process, the density of the photosensitive plate at any particular location is controlled by the shape of the slot, a non-uniform light source will make it difilcult, if not impossible, to obtain the desired tone pattern. Constant speed of the slot is necessary to obtain uniform exposure in one direction of the photosensitive plate. Although theoretically it is only necessary for the slot to expose the negative once, it is desirable to have the slot traverse across the field of the photosensitive plate a number of times to balance any unevenness in the motion of the slot, which would cause irregular exposure in the direction in which constant intensity of the color pattern is desired. It is therefore desirable to employ a combination of light intensity and speed of photosensitive plate which will allow for considerable number of passes of the slot across the plate.

The shape of the lens stop employed in the present invention will vary in accordance with the exposure system employed. Generally, when a negative reverse halftone dot system is employed the lens stop used is revealed to the photosensitive plate as a lighted cross in a dark field, whereas when a positive reverse half-tone system is employed the lens stop will be revealed as a dark cross in a lighted field. The half-tone screen employed is made from two sheets of optical glass, each of which has one surface ruled with equally spaced parallel lines of a definite number to the inch, the two ruled sheets being joined together so that the lines cross at right angles. Each ruled square of the screen reproduces an image of the cross of the lens stop and in this way produces a system of dots. The half-tone screen employed in the process of the present invention is preferably one having from 60 to 150 lines to the inch. The employment of this type of screen will result in a dot pattern which, when transferred to a printing cylinder, will print the dye into the desired uniform pattern onto the plastic interlayer sheeting. The uniformity of the printed sheeting, in some instances, may be further improved by conditioning the printed interlayer sheet until the dye has diffused into the area between the dots. Small changes in intensity are accurately reproduced when screens having from 60 to 150 lines to the inch are employed. The

diameter of the dots formed on the photosensitive The transfer of the image obtained on the negative to the rotogravure cylinder is carried out according to known processes such as the Henderson process described in Us. Patent 2,182,559, wherein a positive reverse halftone dot pattern prepared from the photosensitive plate is interposed between a moving light source and a rotating printing cylinder covered with a photosensitive resist. The motion of the light source and the cylinder is synchronized so that the image of the positive is accurately transferred onto the cylinder, which is then proportionately etched. The etched cylinder is then employed in a rotogravure printing press in which plastic sheeting, such as polyvinyl acetal sheeting, may be printed with a band of dye having a gradation in color intensity corresponding to the gradation in tone density of the photographic pattern. This technique of tinting safety glass interlayer sheeting makes it possible to tint the sheeting continuously and to obtain uniform color intensity in one direction of the sheeting and an identical gradation of the color intensity along all lines perpendicular to the first direction.

I claim:

A method for preparing a photographic plate bearing the image of a tone density pattern, gradated in one direction and uniform in the direction perpendicular thereto which comprises consecutively exposing a photosensitive plate through a focusing lens, a half-tone lens stop, and a half-tone diffraction screen to a uniformly illuminated slot by repeatedly moving said illuminated slot at a uniform speed across the field of the photosensitive plate, the motion of the slot being perpendicular to the longitudinal axis of the slot, the length of said slot being equal to the length of the gradated side of the tone density pattern, the width of said slot varying with the change in tone density desired in accordance with the following equation where W is the width of the slot, the tone density desired and k is the constant determined from a measured density resulting from a known width for the system employed, and thereafter developing the exposed plate to produce a plate containing the desired tone density pattern.

References Cited in the file of this patent UNITED STATES PATENTS 1,896,567 Trist Feb. 7, 1933 OTHER REFERENCES Jones: Photographic Sensitometry, Part I, JSMPE., XVII October 1931, pages 491-535. 

